Projection welding is a resistance welding process which uses heat obtained from resistance to a high electrical current through the work parts held together under pressure by electrodes to effect welding. In so doing a coalescence or welding of the metals is obtained. In spot welding, the size and position of the welds are determined by the size of the electrode tip and the contact point on the workpieces. In projection welding the size and position of the weld or welds are determined by the design of the component to be welded. The welding current and applied force are concentrated in a small, concentrated contact interface formed between the parts to be welded.
In one application of projection welding special nuts, called “weld nuts”, are employed that have projections on the portion of the weld nut to be welded to the sheet metal, also known as the substrate or work piece. The use of projections formed on one face of a pilot nut ensures that a much higher current density is achieved within the small projections, so that the projections are raised to a temperature whereat the metal liquefies. In combination with the pressure from the electrodes sandwiching the nut and substrate together, this causes a strong weld by which the nut is permanently fixed to the substrate.
Weld nuts come in two types. Weld nuts with a protruding cylinder or pilot ring, known as “self-piloted” or “piloted nuts”, are self-aligning to a hole in a work piece into which the piloting ring is fitted. A piloted weld nut has generally a flat, annular side surface on one end and on the other end, a parallel, peripheral, annular outer surface surrounding an inner protruding central cylinder that serves as a pilot ring. The pilot ring is topped by its own flat, annular, pilot ring end face. The annular outer surface surrounding the pilot ring carries the protrusions where welding is to occur.
Non-piloted weld nuts lack the annular pilot ring but otherwise include the protrusions that are necessary for welding. Studs may also be welded to workpiece by resistance welding. While the balance of this disclosure is directed to weld nuts as an example, the invention is equally applicable to the welding of any form of faster and the like to a workpiece.
In carrying-out the welding process a weld nut is held against a metallic work piece, typically at a point where a hole has been formed in the work piece, by a welding electrode head. For purposes of discussion this may be referred to as the “active” electrode, although it is characterized electrically only by the fact that it contacts the weld nut directly. A weld nut can be placed either on the topside surface of the workpiece or on the underside surface of the workpiece. On the other side of the work piece opposite to the location of the nut, a “counter-electrode” completes the electrical circuit to effect the weld. The key objective is to cause the welding current to flow through the weld nut and across the points of its protrusions into the workpiece, so that resistive heating will melt the protrusions and fuse their material with that of the workpiece.
A weld nut must be properly aligned in order to be properly welded to the work piece. To assist in locating the nut with respect to the electrode head, and in locating both the head and nut with respect to a hole formed in a work piece, one of the two electrodes generally contains a centrally mounted alignment pin. This pin passes through the center of an electrode and is generally insulated from the electrode itself by a non-conductive sleeve lining a central bore through the core of the body of the electrode. The electrical isolation of the pin prevents arcing between the tip of the pin in the workpiece and/or a weld nut. Such pins often are arranged to slide within the core of the electrode, permitting them to be advanced for alignment and withdrawn at the moment of welding.
Most typically, this alignment pin is present on the lower of the two electrodes, even in cases where the nut is applied to the workpiece on the topside surface of the workpiece. In this case, the alignment pin extends through the hole formed in the workpiece to penetrate through the hole in the weld nut and assists in the alignment process.
In the past, both active and counter-electrodes have generally been formed of a single, unitary piece. Both such electrodes have been shaped to provide a current-delivery interface at one end that is generally annular in shape. The central opening in this annular shape is occupied by the alignment pin in the case of an active electrode that is placed on the underside of the workpiece, and is penetrated by the tip of the alignment pin in the case of a counter-electrode placed on the upper side of the workpiece. When the active electrode is on the upper side of the workpiece, the alignment pin extends upwardly through a central bore in the counter-electrode and through the hole in the workpiece to penetrate through the open annular central region of the active electrode.
Such prior art active and counter-electrodes have generally been formed of unitary parts made of solid copper. When the annular current delivery interfaces of such electrodes become worn, they must be replaced. Wear at the electrode interface occurs as a result of the vaporization of copper due to arcing. This can form annular grooves in the surface of the annular electrode interface. At a certain stage, such grooves interfere with reliable welding and the electrode heads of the prior art design have to be replaced. As such heads are generally massive, machined to a specific shape and made of copper, the replacement cost is significant.
It would be desirable to provide an improved welding head that can accommodate wear without incurring the full cost of replacing the traditional welding head. This invention addresses that objective.
The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more specific forms will then be further described, and defined, in each of the individual claims that conclude this Specification.